1. Field of the Invention
This invention relates to an improved system for modulating the output from a laser to remove frequency instabilities.
2. Brief Description of the Related Art
It is known that a laser frequency can be stabilized against the frequency of an external reference laser by an automatic feedback control, AFC. This stabilization is achieved by introducing a corrective signal to the laser frequency tuning mechanism by way of a feedback loop. The corrective signal is obtained by reference to a laser frequency signal, for example, from a highly stable low-power CW laser. Various stabilization refinements may be incorporated, including frequency stabilization against a narrow Doppler-free resonance as described in U.S. Pat. No. 3,686,585 to Javan and Freed. However, the radiation frequency of lasers fluctuates, under some conditions, at very high speeds requiring a correspondingly high speed correction of the laser output. The frequency stabilization techniques described above cannot be used if the frequency fluctuation occurs at a rate higher than the response time limit of the laser tuning mechanism. Frequency variations occurring from pulse to pulse and within the pulse may both occur at rates high enough to preclude correction by the laser tuning techniques.
In U.S. Pat. No. 4,329,664 the present inventor describes an adaptive approach to frequency control, ADFC, for the generation of stable frequency radiation at an optical frequency. This approach is different from an AFC method in that it does not utilize a feedback loop. A small sample of the signal from a power laser that produces optical radiation at a frequency, .omega., subject to short-term frequency variations is combined with a signal from a highly stable reference laser to generate a difference beat frequency signal, .omega..sub.m, in the radio frequency range having frequency variations corresponding directly to the fluctuations of the laser frequency The resulting signal represents the instantaneous difference between the primary laser signal and that of the reference laser. This difference signal is amplified and recombined in a modulator with the signal from the primary laser to produce a stable optical signal. The system corrects for both intrapulse chirping, frequency fluctuations within the pulse, and for frequency fluctuations that occur from pulse to pulse.
In one application, the frequency, .omega..sub.o, of an injection-controlled, pulsed CO.sub.2 laser is maintained at an rf offset with respect to the stable reference CW laser at frequency .omega..sub.r. .omega..sub.o is time dependent because of the intrapulse frequency chirp and instabilities. The mixer output voltage, V.sub.b =V.sub.o (t) cos (.delta.t+.phi.), will appear at the rf beat frequency .delta.=.omega..sub.o (t)-.omega..sub.r. The beat-frequency .delta. exactly reproduces the time dependence of .omega..sub.o (t). A CdTe modulator is driven by the beat-voltage signal, after amplification in a broad-band, high-gain amplifier. The two rf sidebands on the IR output of the modulator will appear at .omega..+-.=.omega..sub.o (t).+-..delta.(t). Note, for example, for .omega..sub.r &lt;.omega..sub.o (t), the down-shifted sideband, .omega., exactly reproduces .omega..sub.r, if the pulse from the primary laser is delayed and adjusted to be the same as the delay of the amplified rf beat signal in reaching the modulator, the delays being referenced to the rf mixer input. This time-delay difference, .tau., can readily be adjusted to values below one nsec by adjusting the path-difference in an optical-delay provided at the modulator input. It is desirable to use a broad rf amplifier bandwidth to minimize the rf beat-delay; otherwise a long optical-delay path-length will be needed to correct for the delay.
In the presence of only a small delay-time difference, .tau., the corrected frequency, .omega. in this example, will appear slightly shifted: .omega..sub.- =.omega..sub.r -t dw/dt. For a during-the-pulse chirp rate of, for example, .tau.dw/dt o=200 kHz/usec and .tau..ltoreq.1 nsec, the shift will be .tau.dw/dt o.ltoreq.200 Hz. Since .tau. can be adjusted to be substantially less than 1 nsec, a sizeably higher frequency chirp rate can be tolerated, since this 200 Hz limit is readily permissible in most applications.
This technique removes the entire intrapulse chirp and frequency instabilities to values below 200 Hz in the above example. Inspection shows that the pulse-to-pulse fluctuations of .omega..sub.o (t), are also totally removed from the corrected frequency. For some applications, .tau. as large as tens of nsec can be tolerated, thus considerably relaxing the bandwidth requirements of the rf amplifier.
The adaptively corrected frequency, .omega..sub.c, will appear as the up-shifted (.omega..sub.+) or down-shifted (.omega..sub.-) sideband, depending upon whether .omega..sub.r &gt;.omega..sub.o or .omega..sub.r &lt;.omega..sub.o. With respect to an efficient generation of the sidebands, a near-unity single-sideband conversion efficiency can readily be achieved by driving the modulator at pulsed peak powers in the multi-kilowatt range.